1. Field of the Invention
The present invention relates to a scanning electron microscope which enables to observe a fine object to be measured, the object being located on the surface of a sample. More particularly, the present invention relates to the scanning electron microscope which is arranged to transform the coordinate data of an object to be observed, the object being located on a sample measured by another foreign matter inspector into the data fitted for the coordinate system of itself and then to utilize the converted data when observing an object.
2. Description of the Related Art
A scanning Electron Microscope (often but not always referred to as an SEM) is used for observing a fine structure of a sample in various fields of research and development. The SEM operates to display on screen an SEM image of an object to be observed when observing the object. This technology is applied to observation of a microstructure of a semiconductor device. As a semiconductor device becomes finer, today, the semiconductor device is manufactured with a pattern width of 150 nm or less. In such a semiconductor device, some troubles may take place even if a foreign matter or a defect with as short a width as about several tens nm is located on a wafer used for producing semiconductor patterns. For checking foreign matters or defects that bring about troubles in detail, the SEM is required to observe the foreign matters and the defects. Hereafter, the foreign matters and/or defects are collectively referred simply to as the defects.
For observing such a fine defect with the SEM, it is common to measure where a defect is located on a wafer, search the defect based on the coordinate data obtained by the measurement, and observe the defect through the use of another inspector device such as an optical wafer visual inspector device or an SEM system wafer visual inspector device both of which specify where a defect is located on a wafer with an optical or electron probe. For example, in the case of observing a defect of about 0.2 μm with the SEM, it is necessary to magnify the defect by 5000 times at minimum and display the magnified image of the defect on the SEM screen. However, since the observatory range at one watch is limited by the range of the SEM screen size, if the coordinate data of the defect obtained by the defect inspector device is too much erroneous, the defect may be shifted out of the SEM screen. For example, in the case of observing a defect at a resolution of 10000 times through the use of the SEM with the SEM screen consisting of 150 nm×150 nm, the range of the SEM image to be observed at a watch consists of 15 μm×15 μm. Hence, if the coordinate data sent from the defect inspector device contains an error of ±7.5 μm or more, the defect is shifted out of the SEM screen, so that the defect cannot be found out.
Today, the semiconductor inspector device, representatively the defect review SEM improves its inspecting capability of a defect and thus is required to automatically inspect any kind of defect on any kind of wafer, determine a type of defect, and process data about the defect. In particular, therefore, when observing a defect, if the defect is shifted out of the SEM screen, the process is interrupted or a field of view is moved in the periphery of the SEM screen for inspecting the defect again. Both of the cases need a massive length of time for this inspection. It means that a large number of wafers cannot be inspected efficiently.
Hence, it is necessary to correct the coordinate value of the defect sent from another defect inspector device in consideration of errors such as a difference of a coordinate system of the device, an offset shift of a wafer, a shift of rotation, and a dimensional accuracy error of a coordinate shaft. As a method of correcting the coordinate values, as disclosed in JP-A-11-167893, the SEM inspection takes the method of building correcting expressions with parameters for correcting factors, selecting a plurality of defects used for correction, obtaining the coordinate values sent from the another defect inspector device and the coordinate value on the wafer to be measured, determine the parameters of the correcting expressions from those coordinate values, and correcting all defects located on the wafer.
However, when moving to a location of a defect for obtaining the coordinate value of the defect used for correcting the coordinate value, at an initial state where the coordinate value is not sufficiently corrected, the defect may be shifted out of the view field of the microscope. In particular, when driving the SEM without humans in attendance, if the inspection is executed correctly, the process is interrupted or needs a massive length of time. Hence, the defect cannot be detected efficiently and for a short time.
For preventing the defect from being shifted out of the view field, therefore, frequently, the coordinate values of all the defects are corrected each time a defect is selected and measured. However, the influence of an error taking place in the coordinate value of the defect selected particularly at the initial stage of correction may become an obstruct to the proper correction so that the other defects may be shifted out of the view field. For example, at a first watch, the type of error cannot be grasped, so that the shift amount is subtracted from the coordinate value as the correction.
In a case that the shift amount influences the overall wafer because of shifting the location of the wafer in parallel to the coordinate axis of the wafer coordinate system, by performing the correction of the shift, it is possible to detect the defect without shifting out of the view field at a second or later watch. However, if the shift is brought about by the rotation, the way of shift of the center of the wafer is different from that of the outer periphery of the wafer. Hence, if the first selected point is located on the outer periphery, on the center of the wafer, the defect may be shifted out of the view field. Even if a quite large shift is observed, provision of a plurality of measured coordinate data allows the observed point to be excluded for improving accuracy. At an initial stage, however, it cannot be grasped that the shift amount to be measured is exceptional on the overall wafer.
As described above, at the initial state of correction, the trend for a correction of the coordinate value cannot be grasped and thus the correction is executed in light of the coordinate value. Hence, if the error is extremely large, the correction may put the defect out of the view field. For correcting the coordinate values of all the defects located on the wafer, it is necessary to select the defect used for the correction from the overall wafer uniformly or without shifting to a specific area. Hence, in some methods of selecting a defect, the SEM is often moved on the wafer between the ends, which needs a long time for correcting the coordinate value.
In consideration of the foregoing disadvantages, it is an object of the present invention to provide a method of selecting a defect which method is arranged to simply and efficiently transform the coordinate data of a defect sent from a defect inspector device into the coordinate data of an SEM.